The present invention relates to surface-coated steel strips which can exhibit stable and uniform color tones as well as improved resistance to corrosion.
More particularly, the present invention relates to surface-coated steel strips especially suitable for use as painting-free steel strips, i.e. unpainted steel strips.
Recently, in the production of home electrical appliances, office automation equipment, automobiles and the like, there has been a tendency to reduce manufacturing costs by using unpainted steel materials. These steel materials in the form of steel strips, steel sheets, steel pipes, shaped steels, and the like must have better resistance to corrosion and a better surface appearance than conventional painted steel materials.
So far, several corrosion resistant steel materials have been proposed:
(1) Zn-alloy plated steel strips, e.g., Zn-Co, Zn-Ni, and Zn-Co-Cr alloy plated steel strips, or Zn-alloy+chromate film-coated steel strips. Such materials are disclosed in Japanese patent application Laid-Open Specification No. 1986/1974.
(2) Zn-plated and chromate film-coated steel strips with an organic resin film or silicate film. Such materials are disclosed in Japanese patent application Laid-Open Specification No. 108292/1982.
Since these steel materials are grayish white or yellowish green in color, they have only limited use as unpainted steel strips due to their surface appearance.
Furthermore, it is known in the art that a Zn-alloy coating is effective to prevent the formation of red rust, but not to prevent white rust. In addition, when a chromate solution of the reaction type is applied to a Zn-alloy, the Cr deposit amounts to only a few milligrams per square meter. Such a small amount of a deposit is not effective to prevent white rust. In case of a chromate solution of the coating type, which is coated through roll coaters, spray nozzles, etc., it is possible to apply a large amount of Cr with an improvement in resistance to white rust. However, it is difficult to apply the coating uniformly, the surface appearance is not good enough to use without painting, and a problem of dissolution of free Cr.sup.6+ ions is inevitable. This restricts application of chromate coated steel strips as painting-free steel strips.
On the other hand, there are also a number of steel materials which are known for having a good surface appearance. Examples of these materials are as follows:
(i) Pre-painted steel strips, or steel strips painted after formation.
(ii) Steel strips with a black resin film mainly containing carbon black. Such materials are disclosed in Japanese patent application Laid-Open Specification No. 62996/1981.
(iii) Black steel strips with a precious metal-containing chromate film.
(iv) Colored steel strips which are colored by dipping into a hydrochloride or molybdate solution.
(v) Electroplated steel strips using a Zn-plating bath containing Co ions, Ni ions, and other additives, the electroplated steel plates being subjected to an anodic treatment to color the strip surface. Such materials are disclosed in Japanese patent application Laid-Open Specification No. 151490/1983 and No. 151491/1983. More specifically, Japanese patent application Laid-Open Specification No. 151491/1983 discloses a steel strip which has an electroplated coating of a zinc alloy and an anodic treated layer. If necessary, an additional treatment such as a chromate treatment of the coating type and a resin coating treatment may be applied thereto.
However, all of the above materials suffer from various problems. Pre-painted steel strips are quite expensive and they cannot be welded. Black resin-coated steel strips exhibit poor resistance to white rust. Black chromate-coated steel strips have poor corrosion resistance, and as they contain a precious metal such as Ag, the material cost is inevitably high. In the case of colored steel strips which are colored using dipping bath, it is difficult to control the bath composition, and it takes a long time to obtain a stable surface appearance.
Steel strips which are subjected to electroplating and then to anodic treatment do not exhibit a deep color, and the surface appearance is often ununiform due to the influence of the flow rate of the electrolyte solution during plating and anodic treatment.
Furthermore, an anodized surface layer comprises oxides of metals such as Co, Ni, and Mo in the form of a porous film having pores on the order of an Angstrom. Therefore, it has poor resistance to white rust. Application of a silicate film has also been proposed, but steel strips with a silicate film do not have good resistance to white rust.
U.S. Pat. No. 4,548,868 discloses a surface coated steel strip comprising a steel strip, a zinc alloy layer electroplated on the strip, a chromate film formed on the zinc alloy layer, and a polyethylene coating cured to the chromate film. The chromate film is formed by so-called coating type method, and a substantial amount of free Cr.sup.6+ ions inevitably remains in the film.